Congrats on the new Cummins and welcome to the site! There really isn't anything that you can do at this point to get better mileage that you are already seeing. Since your '13 uses DEF for the REGENS you are already getting better mileage than i am with my '12. At least you have the benefit of your MPGs getting better as the engine breaks in.
The typical mods like intake and exhaust don't add much if any MPG gains on these trucks since they already have a free flow design. Adding a tuner could possibly help a little but i am not aware of anyone that has cracked the codes for the '13 diesels yet. and adding tuner adds the possibility of the dealer voiding your warranty. Disabling the EGR would help your MPGs but that is not really an option without a race tuner. Disabling without a tuner will throw a CEL (check engine light) and set the truck in limp mode.
That being said, these trucks look awesome lifted!! Can't wait to see what you do with yours. Would love to see pics when you get a chance
